The present invention relates to steerable nozzles for rocket engines. The field of application of the invention is more particularly, but not exclusively, that of missiles, in particular tactical missiles having a diameter of less than about 500 millimeters (mm) and intended for operation under varying conditions of external pressure. A typical example of such a missile is a tactical missile launched from a submarine. The missile is ignited at great depth (about 150 meters (m)) and terminates its operational lifetime in the atmosphere after it has followed a determined trajectory obtained by controlling its nozzle(s).
The technology that makes use of a moving diverging portion for steering thrust presents numerous advantages compared with other known technologies for steering thrust such as those using a steerable nozzle on a flexible abutment, for example. Nozzles in which only the diverging portion is movable present thrust deflection capacities that are much greater than those which can be obtained with steerable nozzles in which the entire nozzle flow section is movable. This improvement in thrust deflection is explained by the fact that when the moving zone of the diverging portion is in a swivelled configuration, there exists an asymmetrical pressure field which leads to an amplification coefficient that is greater than 1 compared with the geometrical effect on its own. This technology and its advantages compared with other types of steerable nozzle are described in particular in French patent application FR 02/08370.
Nevertheless, such controllability can be obtained only on the assumption of not disturbing the flow in the flow section passing through the nozzle, where such a disturbance would be associated with the external pressure and the nature of the medium (liquid or gas). Unfortunately, given the combustion pressure levels presently generated in thrusters and the expansion profiles adapted for optimizing thrust thereof over the entire duration of a mission, the jet in the “active” thrust-deflection zone of the diverging portion can become separated in the presence of an external pressure that is high.
This phenomenon is shown in FIG. 1 which is a highly diagrammatic view of a rocket engine fitted with a nozzle having a moving diverging portion. The rocket engine comprises a casing 10 surrounding a combustion chamber 11 which opens out into a nozzle. The nozzle 20 is formed by a throat 12 receiving the hot gas produced in the combustion chamber, and by a diverging portion 20. The diverging portion 20 is movably mounted on the throat 12, which is itself secured to the end of the casing 10. Thus, the diverging portion is the only moving portion of the nozzle and by pivoting it enables the jet of combustion gas coming from the throat to be deflected so as to steer the trajectory of the thruster by changing the direction of its thrust vector.
Under conditions of high external pressure, as occurs under the sea, the diameter D of the jet 32 from the nozzle shrinks, causing the jet to become separated from the wall of the diverging portion, as shown in FIG. 1. This comes from the fact that nozzles are generally optimized as a function of the overall performance of the launcher. For this purpose, an expansion profile (i.e. the variation in the section of the nozzle) is defined as a function of a certain altitude referred to as the “matched” altitude, above which the majority of the flight is situated. Below this altitude, the nozzle is overexpanded and the high external pressure can lead to the jet becoming separated from the wall of the diverging portion.
When the jet separates from the wall of the diverging portion, a disturbed zone 30 is formed which extends from the point of jet separation to the end of the diverging portion, and into which the external fluid is drawn, which fluid can be a liquid or a gas, depending on the medium. The zone 31 against which pressure forces are applied, which leads, on swivelling, to a lateral force for steering the trajectory of the missile, and which usually extends over the entire inside surface of the diverging portion, becomes restricted to the upstream fraction of the diverging portion, i.e. to the fraction between the outlet from the throat and the point at which the jet separates inside the diverging portion. The asymmetry of the pressure field inside the diverging portion is considerably reduced, thereby reducing the lateral force that steers the missile. The influence of the lateral force on missile steering is even more limited since the zone of force application is located close to the point where the diverging portion bears against the throat (reduced lever effect). Thus, so long as the jet is separated from the wall of the diverging portion, the controllability of the nozzle remains very limited.
Furthermore, separation of the jet inside the nozzle can lead to instabilities causing vibratory stresses that are mechanically harmful.
The problem of reduced controllability due to the jet separating inside the diverging portion is particularly awkward in nozzles having a moving diverging portion since it is the forces that are applied against the inside wall of the diverging portion that enables the missile to be controlled in flight. In other systems using steerable nozzles, such as those including nozzles with a flexible abutment, it is the entire stream from the nozzle that is moved. Thus, the jet of gas ejected from the combustion chamber is deflected directly on leaving the throat since the axis of the throat is offset together with that of the diverging portion. Consequently, the thrust vector is steered at the throat of the nozzle, i.e. upstream from the diverging portion, so separation of the jet inside the diverging portion then has practically no influence on the controllability of the nozzle.